#include <stdio.h>
#define HAPPY_FACTOR 3
void main(void)
{
    // #if...#endif指令用于预处理器的条件判断，满足条件时，内部的行会被编译，否则就被编译器忽略。
    #if 0
    const double pi = 3.1415; // 不会执行
    #endif
    
    // 对于没有定义的宏就是0
    #if HAPPY_FACTOR == 0
        printf("HAPPY_FACTOR is 0!\n");
    #elif HAPPY_FACTOR == 1
        printf("HAPPY_FACTOR is 1!\n");
    #else
        printf("HAPPY_FACTOR is other !\n");
    #endif

  
}